The type of blood monitoring systems to which the invention pertains has been widely used to monitor a patient's hematocrit and oxygen saturation levels during conventional hemodialysis treatments. Patients with kidney failure or partial kidney failure typically undergo hemodialysis treatment in order to remove toxins and excess fluids from their blood. To do this, blood is taken from a patient through an intake needle or catheter which draws blood from an artery or vein located in a specifically accepted access location (for example, a shunt surgically placed in an arm, thigh, subclavian, etc.). The needle or catheter is connected to extracorporeal tubing that is fed to a peristaltic pump and then to a dialyzer that cleans the blood and removes excess water. The cleaned blood is then returned to the patient through additional extracorporeal tubing and another needle or catheter. Sometimes, a heparin drip is located in the hemodialysis loop to prevent the blood from coagulating. By way of background, as the drawn blood passes through the dialyzer, it travels in straw-like tubes within the dialyzer which serve as semi-permeable passageways for the unclean blood. Fresh dialysate solution enters the dialyzer at its downstream end. The dialysate surrounds the straw-like tubes and flows through the dialyzer in the opposite direction of the blood flowing through the tubes. Fresh dialysate collects toxins passing through the straw-like tubes by diffusion and excess fluids in the blood by ultra filtration. Dialysate containing the removed toxins and excess fluids is disposed of as waste. The red cells remain in the straw-like tubes and their volume count is unaffected by the process.
It is known in the art to use an optical blood monitoring system during hemodialysis, such as the CRIT-LINE® monitoring system sold by the assignee of this application. The current CRIT-LINE® blood monitoring system uses optical techniques to non-invasively measure in real-time the hematocrit and the oxygen saturation level of blood flowing through a hemodialysis system or other systems involving extracorporeal blood flow. When the CRIT-LINE® system is used with conventional hemodialysis systems, a sterile, single-use blood chamber is usually attached in-line to the extracorporeal tubing on the arterial side of the dialyzer.
In general, blood chambers along with the tube set and dialyzer are replaced for each patient and the blood chamber is intended for a single use. The blood chamber provides an internal blood flow cavity, a substantially flat viewing region and two viewing lenses. Blood chambers commonly used are molded from clear, medical-grade polycarbonate. Typically, one of the viewing lenses is integrally molded with the body of the polycarbonate blood chamber, and the other viewing lens is molded into a separate lens body that is sonically welded or otherwise fixed to the chamber body. Alternatively, both lenses are molded into separate lens bodies that may be welded or otherwise affixed into place on the chamber body.
LED emitters and photodetectors for the optical blood monitor are clipped into place onto the blood chamber over the lenses. Multiple wavelengths of light may be directed through the blood chamber and the patient's blood flowing through the chamber with a photodetector detecting the resulting intensity of each wavelength. The preferred wavelengths to measure hematocrit are about 810 nm (e.g. 829 nm), which is substantially isobestic for red blood cells, and about 1300 nm, which is substantially isobestic for water. A ratiometric technique implemented in the CRIT-LINE® controller, substantially as disclosed in U.S. Pat. No. 5,372,136 entitled “System and Method for Non-Invasive Hematocrit Monitoring”, which issued on Dec. 13, 1999 and is assigned to the assignee of the present application, uses this information to calculate the patient's hematocrit value in real-time. The hematocrit value, as is widely used in the art, is the percentage determined by dividing the volume of the red blood cells in a given whole blood sample by the overall volume of the blood sample.
In a clinical setting, the actual percentage change in blood volume occurring during hemodialysis can be determined, in real-time, from the change in the measured hematocrit. Thus, an optical blood monitor, such as the CRIT-LINE® monitor, is able to non-invasively monitor not only the patient's hematocrit level but also the change in the patient's blood volume in real-time during a hemodialysis treatment session. The ability to monitor real-time change in blood volume helps facilitate safe, effective hemodialysis.
The mathematical ratiometric model for determining the hematocrit (HCT) value can be represented by the following equation:
                              H          ⁢                                          ⁢          C          ⁢                                          ⁢          T                =                  f          ⁡                      [                                          ln                ⁡                                  (                                                            i                                              λ                        ⁢                                                                                                  ⁢                        2                                                                                    I                                              0                        -                                                  λ                          ⁢                                                                                                          ⁢                          2                                                                                                      )                                                            ln                ⁡                                  (                                                            i                      λ1                                                              I                                              0                        -                                                  λ                          ⁢                                                                                                          ⁢                          1                                                                                                      )                                                      ]                                              Eq        .                                  ⁢                  (          1          )                    where iλ2 is the infrared light intensity detected by the photoreceiver at about 810 nm, iλ1 is the infrared intensity detected at 1300 nm and I0-λ2 and I0-λ1 are constants representing the infrared light intensity incident on the blood accounting for losses through the blood chamber. The function ƒ[ ] is a mathematical function which has been determined based on experimental data to yield the hematocrit value. Preferably, the function ƒ[ ] in the above Equation (1) is a relatively simply polynomial, e.g. a second order polynomial. The above Equation (1) holds true only if the distance traveled by the infrared light radiation from the LED emitters to the photodetectors at both wavelengths are constant distances and preferably the same distance
The preferred wavelengths to measure oxygen saturation level are about 810 nm and about 660 nm. The mathematical ratiometric model for determining oxygen saturation level (SAT) can be represented by the following equation:
                              S          ⁢                                          ⁢          A          ⁢                                          ⁢          T                =                  g          ⁡                      [                                          ln                ⁡                                  (                                                            i                                              λ                        ⁢                                                                                                  ⁢                        3                                                                                    I                                              0                        -                                                  λ                          ⁢                                                                                                          ⁢                          3                                                                                                      )                                                            ln                ⁡                                  (                                                            i                      λ1                                                              I                                              0                        -                                                  λ                          ⁢                                                                                                          ⁢                          1                                                                                                      )                                                      ]                                              Eq        .                                  ⁢                  (          2          )                    where iλ3 is the light intensity of the photoreceiver at 660 nm, iλ1 is the detected intensity at 810 nm and I0-λ3 and I0-λ1 are constants representing the intensity incident on the blood accounting for losses through the blood chamber. The function g[ ] is a mathematical function determined based on experimental data to yield the oxygen saturation level, again preferably a second order polynomial. Also, like Equation (1) for the hematocrit calculation, Equation (2) for the oxygen saturation level calculation holds true only if the distance traveled by the visible and infrared light from the respective LED emitter to the respective detector at both the 660 nm and 810 nm wavelengths are constant distances and preferably the same distance.
In the art, the LED emitters and the photodetectors are mounted on a sensor clip assembly. For accuracy of the system, it is important that the LED photoemitters and the photodetectors be located in a predetermined position and orientation each time the sensor clip assembly is clipped into place over the blood chamber. The optical monitor is in fact calibrated for the specific dimensions of the blood chamber and the specific position and orientation of the sensor clip assembly with respect to the blood chamber. For this purpose, in the prior art, the heads of the sensor clips are designed to mate in a fixed orientation with non-circular, raised and stepped rims surrounding the viewing lenses on the blood chamber (e.g. double-D configuration). More specifically, the heads on both sides of the sensor clip assembly are formed in a non-circular shape, e.g. a double-D configuration, which matches the corresponding non-circular shape of the raised, stepped rims surrounding the viewing lenses on the blood chamber so that the sensor clip heads fit on the blood chamber in a fixed orientation and are prevented from rotating relative to the blood chamber. While the double-D configuration has proven to work well, one drawback of the design is the additional amount of medical grade polycarbonate material that is required to manufacture the raised, stepped rims. In order to reduce the cost of manufacturing the blood chambers which are single-use, disposable medical devices, it is desirable to reduce the amount of medical grade polycarbonate in the blood chambers.
If not addressed properly, stray ambient light and light piping through the blood chamber can cause serious inaccuracies in the measured hematocrit and/or oxygen saturation levels. Sophisticated signal processing techniques have been used in the art to remedy most of the issues pertaining to ambient light. In addition, prior art blood chambers are molded with a moat around a relatively thin, flat viewing area in the blood flow cavity between the viewing lenses. This internal moat within the blood flow cavity fills with blood and blocks light from the silicon and gallium indium arsenide photodetectors on the sensor clip assembly unless the light propagates on a direct path from the respective LED emitter, through the blood in the blood flow cavity, to the respective photodetector. The effectiveness of the moat depends on many factors including the patient's hematocrit level and the wavelength spectrum of the light that is sought to be blocked from the photodetectors. In practice, the above-mentioned signal processing techniques have been found necessary to cope with most ambient light issues, whereas the moat has been found useful to reduce inaccuracies due to light piping in most circumstances. Co-pending patent application Ser. No. 12/876,572, entitled “Blood Chamber for an Optical Blood Monitoring System”, by Barrett et al, assigned to assignee of the present application and incorporated herein by reference, discloses the use of an opaque chamber body in order to prevent inaccuracies when measuring oxygen saturation levels due to light ducting which can occur at low oxygen saturation levels and low hematocrit levels. Both the use of the moat and the opaque chamber body physically block piped and/or ambient light. The present invention is directed to providing another way to physically block ambient light from the photodetectors.